Most caps are pretty much universal. You just get one the correct size (5/50) and the correct voltage rating (370 VAC or higher). They can be either oval or cylindrical in shape. As to the cost, the last one I bought (myself, not through a contractor) was ~$20. In fact, the one that I bought is nearly new and I don't need it any more (just installed a new heat pump). If the ratings are correct, I'd be happy to send to you. I can't remember if mine is 5/35 or 5/50. I do know it is 370 VAC. I'll check tonight and let you know.

They are pretty easy to change. Make sure there is no power to the unit (pull the disconnect) and take a couple pics of it before you do anything (so you don't forget how the wires go). Slide the wires off the old cap and slide on the new one. Then mount the new cap in the holder. Probably only take you 10 minutes to do the whole thing. However, I would probably just keep the new one on hand and wait until it is needed. The other option is to switch them and keep the old as a spare. This way if the new one blows, you won't be stuck on a really hot/cold day waiting for a part or paying big $$$ on an emergency call to get someone to fix it.

You could try a local HVAC supplier, but they may not sell to you. Places like Grainger can get them, but you would need to be a contractor or have an account. You should have an easy time picking one up online, though.

I'm not convinced that there is actually anything wrong with your cap, but a spare wouldn't hurt. I don't recall what the tolerances on these capacitors are, but many large caps are typically +/- 10% or 20%. At +/- 10%, the cap should read between 45 and 55. At +/- 20%, the cap would be between 40 and 60.

Some capacitors that are used for power supplies in electronics may have ratings like -20%/+80%. The reason is that in this case, too big isn't a problem and is better than being too small.

There aren't a lot of capacitors out there that are better than +/- 10%. The ones that are out there are usually smaller caps and are ones where getting the value close to exact is important.

My point is that a 43 measured out of a 50 nominal value may not mean a thing except the HVAC guy maybe looking to make some extra $$$ off you.

BTW: If they said that part about being "less than 90% efficiency" they either don't know what they are talking about or are trying to empty your wallet. If I had a cap that measured 55, but was rated at 50, I wouldn't say it is "110% efficient". All it means is it came out of the factory with 10% more storage capacity than what the sticker says (which doesn't change a thing on how your HVAC system operates).